Issues with my new QSC PLX 3002

I originally purchased this amp so I could run it at approximately the same wattage (1100) stereo as I was running my Carvin DCM1000 bridged mono (1000 watts). Today, I finally AB'd both amps run in stereo mode. I fully expected the QSC to be louder than the Carvin (550 watts into 8 ohms), but the Carvin is definitely louder in stereo even though it's rated at only about 200 watts at 8 ohms. What gives? Is it the input sensitivity issue again?

Another problem: The amp has some bad rattles inside when it's setting on top of my cabinet. Is this normal for these amps? My Carvin is dead quiet; the fan is quieter too.

Well, is this amp really worth about 3 times as much as my Carvin? Should I sell this thing right away and keep the Carvin?

My first 3002 I returned b/c of the rattling issue. The new one is better, but still not totally quiet. I've put some foam b/t the cabs and the rack which eliminates all vibration/rattles. As far as power goes, I haven't noticed an overwelming increase over my SM400s (in bridged mono), but I can now run my aggies at 8ohms.

I can't speak to the volume issues. On the rattling, however, I can offer some advice. I've been using a PLX 1202 for several months and I was also plauged with a lot of vibration issues and rattling when I placed my rack containing the power amp directly on top of my 2 GS112's. To solve the problem I bought a set of these:

The MoPads work marvelously. No more rattling issues at all. I highly reccommend them. Even if I never would have had the rattling problems I know I've read many times that the constant vibrations that a cabinet can place on pre/power amp components is not good over a long period of time regardless of rattling or not. The MoPads will completely isolate your rack from your cabs. You can get the MoPads at your local GC for around $30.00.

I originally purchased this amp so I could run it at approximately the same wattage (1100) stereo as I was running my Carvin DCM1000 bridged mono (1000 watts). Today, I finally AB'd both amps run in stereo mode. I fully expected the QSC to be louder than the Carvin (550 watts into 8 ohms), but the Carvin is definitely louder in stereo even though it's rated at only about 200 watts at 8 ohms. What gives? Is it the input sensitivity issue again?

Another problem: The amp has some bad rattles inside when it's setting on top of my cabinet. Is this normal for these amps? My Carvin is dead quiet; the fan is quieter too.

Well, is this amp really worth about 3 times as much as my Carvin? Should I sell this thing right away and keep the Carvin?

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Because it is more powerful, the amp is capable of being louder if you acually push it to doing so. If you make the Carvin put out, say, 50 watts and the PLX 20 watts, then yes, the Carvin will be louder.

It's more an issue of gain than input sensitivity.

You didn't mention anything about your gain settings. Have you tried manipulating them?

Thanks for the replies. I do have an Auralex GRAMMA pad that I might just put in between the amp and speaker cabinet.

Bob,

What gain settings are you talking about? I have the gain all the way up on the power amp. I AB'd them again today using the 1/4" output from my Sansamp, and they seemed closer in volume. I still don't like this rattle in the QSC, though. What inside the amp is doing that? Can something be tightened down or muted inside? I'm about ready to pry inside and see what's going on.

From here I can't tell what is causing the rattle , and even when the amp is right close by, rattles don't always come from where they seem to.

Within the past couple weeks I advised another bassist/PLX user, who thought that the air duct over the heat sinks might've been rattling, to wedge some rubber feet or something similar between it and the reservoir capacitors. He did that, put it back together and still had the rattle, and it turned out to be actually the chassis of the next device in the rack rattling against the chassis of the PLX.

Within the past couple weeks I advised another bassist/PLX user, who thought that the air duct over the heat sinks might've been rattling, to wedge some rubber feet or something similar between it and the reservoir capacitors. He did that, put it back together and still had the rattle, and it turned out to be actually the chassis of the next device in the rack rattling against the chassis of the PLX.

For what it's worth, here's the Advice that Bob gave me in My Thread regarding the vibration I was experiencing. It's on the 6th page.

Bob Lee (QSC) said:

It's possible that the fishpaper duct over the heat sinks might rattle. It's hard for me to tell what's doing it without actually finding the source of the noise. Some reportedly do, but I haven't ever seen or heard it for myself on the various PLX amps I've used and played around with.

Try tapping on the sides of the duct and see if it hits against something in a way that could cause a rattle. If you want to try wedging something between the side of the duct and the reservoir capacitors, go ahead--maybe some weatherstripping foam, some adhesive rubber feet, or such. It won't hurt anything, as long as it won't fall out.

I've used my PLX 1602 in a rack case on top of my bass cabinet for close to six years. Its a pre-production beta unit, and the duct is floppier than the ducts used in regular production, but it's never rattled.

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BTW, you need to open the bottom of the amp. screws are on the back and sides.

I've isolated the problem to the smaller heatsink attached to the circuit board on the front left side when looking at the opened power amp from the bottom. Right now my idea is to take a strong rubber band and wind it around the heatsink and one of the bigger blue capacitors to the right of it. I'm just wondering how hot these things actually get.

Gee, would this void my warranty? More important, is this something that is covered under the warranty? The unit's about 1 year old.

Because it is more powerful, the amp is capable of being louder if you acually push it to doing so. If you make the Carvin put out, say, 50 watts and the PLX 20 watts, then yes, the Carvin will be louder.

It's more an issue of gain than input sensitivity.

You didn't mention anything about your gain settings. Have you tried manipulating them?

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+1. Can I drop in the controversial car analogy at this point?

What's faster. A Toyota driven at 80pmh, or a Ferrari driven at 60mph?

My bet is that a voltmeter will show that the QSC was putting out less power at the same input gain. Adjust the gain so both amps have the same output and the volume issue will solve itself.

You clearly don't understand the terminology. For example a gain of 2 means that whatever signal is on the input appears at the output twice as large.

Now back to the two amps, lets say they are both set for a gain of 10. Are you saying that with a 1 volt signal on each of their inputs one amp will put out a 10 volt signal (1*10=10) and the other somthing other than 10 volts (1*10 Not-equal to 10)?

You clearly don't understand the terminology. For example a gain of 2 means that what signal is on the input appears at the output twice as large.

Now back to the two amps, lets say they are both set for a gain of 10. Are you saying that with a 1 volt signal on each of their inputs one amp will put out a 10 volt signal and the other somthing other than 10 volts?

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That's certainly how it was explained to me in other TB threads. I've never built an amp and I don't profess to be an expert, but from listening to what the experts on TB say, every amp has a different sensitivity to that 1v input. Lets pretend they're both 1000w amps. Sure they both produce 1000w at the onset of clipping, but both will need a different amount of input level to achieve the maxiumum 1000w. Therefore pumping the the identical input signal will produce a different output for each amp.